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1. Field of the Invention
The present invention relates to the treatment of ores and ore concentrates to recover metal values, and in particular relates to the pressure oxidation treatment of sulfide ores and ore concentrates to enable the recovery of precious metal values including silver.
2. Description of Related Art
Silver is a valuable precious metal and can be found in precious metal ores such as acanthite (Ag2S). In addition, precious metals such as silver and gold are also found associated with other sulfide-containing ores.
There are many hydrometallurgical processes available for the treatment of silver-bearing sulfide ores to recover non-ferrous metal values (e.g., copper) as well as any gold that may be associated with the ore. However, the silver can be difficult to recover in an economically feasible manner using these processes.
Hydrometallurgical processes are generally preferred over methods such as smelting due to the environmental issues associated with smelting sulfide ores. Pressure oxidation is one known hydrometallurgical process for recovering metals from sulfide-containing ores and ore concentrates. During pressure oxidation, a slurry including the ore is subjected to elevated pressure and temperature while in contact with oxygen to decompose the minerals. The sulfide components of the ore are at least partially oxidized, liberating metals. The metals can then be recovered from the solids and/or the solution of the discharge slurry.
U.S. Pat. No. 5,698,170 by King discloses a method for the pressure oxidation of a copper-containing material followed by solvent extraction and electrowinning (SX/EW) to recover copper. The pressure oxidation step produces a high acid content solution, which is diluted after the pressure oxidation step and prior to recovery of the copper in a SX/EW circuit.
One of the problems associated with pressure oxidation of sulfide ores that also include iron is the formation of jarosite compounds. In particular, certain metals that can be found in the ore, including silver, preferentially form jarosite compounds during pressure oxidation. When the silver is associated with a jarosite compound, the silver is difficult to recover in an economical manner.
The article entitled xe2x80x9cPressure Oxidation of Silver-Bearing Sulfide Flotation Concentratesxe2x80x9d by Thompson et al., (published in Mining Engineering, September 1993, pp. 1195-2000) discloses the pressure oxidation of sulfide flotation concentrates at a temperature of 160xc2x0 C. to 225xc2x0 C. It is disclosed that most of the silver in the autoclaved solids is associated with jarosites that are formed by hydrolysis of ferric sulfate. The silver associated with these jarosites is extremely refractory to cyanide leach treatment resulting in silver extractions of less than 5 percent. In order to recover higher levels of silver, the jarosites must be decomposed at an elevated temperature in the presence of lime (CaO), a process commonly referred to as a xe2x80x9clime boil.xe2x80x9d However, a lime boil uses excessive quantities of lime, often in excess of 400 lbs. per ton of autoclaved solids, and adds significantly to the cost associated with recovering the silver.
U.S. Pat. No. 5,096,486 by Anderson et al. discloses a process for extracting silver from silver sulfide bearing solids by leaching a metal bearing mineral with an aqueous liquid including sulfuric acid and sodium nitrite. The silver is solubilized and is recovered from pressure oxidation discharge solution by precipitating silver chloride. However, sodium nitrite forms nitric acid and the associated off-gases are extremely harmful, if discharged, to the environment. It is also disclosed that maintaining 115 g/l or more of sulfuric acid in the aqueous mixture of sulfuric and sodium nitrite will prevent the formation of argentojarosite and plumbojarosite.
It would be useful to provide a method for treating silver-bearing sulfide ore and/or sulfide ore concentrates by pressure oxidation such that the silver is not combined in substantial quantities with refractory minerals such as jarosite and such that the silver is amenable to extraction from the solids portion using conventional cyanide leach methods without the need for a jarosite destruction step.
The present invention is directed to the pressure oxidation of a mineral feed that includes at least iron, sulfide sulfur and silver wherein the pressure oxidation conditions are controlled to reduce the formation of jarosite mineral species in the solids portion of the discharge slurry.
During pressure oxidation of sulfide minerals according to the prior art, particularly those including iron, substantial quantities of jarosite compounds are typically formed and discharged from the pressure oxidation reactor in the solids portion of the discharge slurry. Equations 1 and 2 are representative of the reactions that are believed to normally occur in the formation of jarosite from pyrite during pressure oxidation.
4FeS2+15O2+5H2Oxe2x86x92Fe2(SO4)3+Fe2O3+5H2SO4xe2x80x83xe2x80x83(1)
3Fe2(SO4)3+14H2Oxe2x86x922(H3O)Fe3(SO4)2(OH)6+5H2SO4xe2x80x83xe2x80x83(2)
Various metals and functional groups found in the mineral feed can substitute for the hydronium (H3O) group in the jarosite, including potassium (K), sodium (Na), rubidium (Rb), silver (Ag), thailium (TI), ammonium (NH4), lead (Pb) and mercury (Hg). When silver-containing jarosite species form, silver metal is very difficult to recover using conventional leaching methods without first subjecting the solids to a jarosite destruction step such as a lime boil.
In accordance with the present invention, the formation of jarosite species can be substantially inhibited by careful control over the pressure oxidation conditions. One way to control the pressure oxidation conditions is through the addition of a sulfate-binding material to the pressure oxidation step. The reactions that are believed to occur during the pressure oxidation step according to this embodiment of the present invention, when using calcium in the form of calcium carbonate as the sulfate-binding material, are illustrated by Equations 3, 4 and 5.
xe2x80x834FeS2+15O2+8H2Oxe2x86x922Fe2O3+8H2SO4xe2x80x83xe2x80x83(3)
CaCO3+H2SO4+H2Oxe2x86x92CaSO4.2H2O+CO2xe2x80x83xe2x80x83(4)
CaSO4.2H2Oxe2x86x92CaSO4+2H2Oxe2x80x83xe2x80x83(5)
As is illustrated by Equation 4, the added calcium from the calcium carbonate preferentially binds sulfate by forming calcium sulfate and inhibits the formation of other sulfate species, such as jarosites and iron sulfate. The iron is converted to insoluble hematite (Fe2O3) and therefore the amount of iron solubilized in the discharge liquid is also reduced. The silver, which under typical pressure oxidation conditions would be associated with jarosite, is precipitated as elemental silver, silver sulfide and/or silver inclusions in hematite, all of which are now recoverable in a standard leaching step without the need for a lime boil or similar jarosite destruction step.
When calcium is used as the sulfate-binding material in the form of a calcium compound such as calcium carbonate, most of the calcium crystallizes to form crystalline anhydrite (CaSO4) in the discharge solids, which is more amenable to thickening and/or filtration than gypsum (CaSO4.2H2O). The conversion of most of the iron to hematite in the solids portion of the discharge slurry also simplifies filtration and other downstream processing steps that may be used.
Thus, according to one embodiment of the present invention, a method for processing a mineral feed comprising iron, sulfide sulfur and silver to facilitate recovery of silver is provided. The method includes the steps of: pressure oxidizing an aqueous feed slurry that includes the mineral feed wherein at least about 70 percent of sulfide sulfur in the mineral feed is converted to sulfate sulfur; recovering from the pressure oxidizing step an aqueous discharge slurry comprising discharge solids and aqueous discharge liquid, the discharge solids comprising at least a portion of the silver and at least a portion of the iron from the mineral feed; and leaching at least a portion of the discharge solids with a leach solution to dissolve into the leach solution at least a portion of the silver from the discharge solids. Preferably, the concentration of dissolved iron in the discharge slurry is not greater than 1 gram of dissolved iron per liter of aqueous discharge liquid. Advantageously, the method of the present invention can be practiced without the use of a jarosite destruction step between the pressure oxidizing step and the leaching step.
According to another embodiment of the present invention, a method for the treatment of a mineral feed comprising iron, sulfide sulfur and silver is provided. The method includes the steps of pressure oxidizing an aqueous feed slurry including the mineral feed at a temperature of at least about 160xc2x0 C. and withdrawing a discharge slurry from the pressure oxidation step that includes discharge solids and a discharge liquid, wherein the pressure oxidizing step is conducted in the presence of a sufficient concentration of a sulfate-binding material such that at least about 75 wt. % of the silver contained in the mineral feed is discharged in the discharge solids and not greater than 25 wt. % of the silver contained in the discharge solids is associated with jarosite species. Preferably, the sulfate-binding material is in the form of a compound selected from the group consisting of carbonates, hydroxides and oxides of metals selected from the group consisting of calcium, sodium, potassium and magnesium.
According to another embodiment, a method for recovering silver from a mineral feed comprising sulfide sulfur, iron and silver is provided. The method can include the steps of pressure oxidizing an aqueous slurry comprising the mineral feed in the presence of oxygen gas to convert at least 80 percent of the sulfide sulfur in the mineral feed to sulfate sulfur, the pressure oxidizing step being conducted at a temperature of at least 210xc2x0 C. Discharge solids are recovered from the pressure oxidizing step, the discharge solids comprising at least a portion of the iron and a portion of the silver from the mineral feed and at least a portion of the silver is leached from the discharge solids recovered from the pressure oxidizing step wherein not greater than 25 wt. % of the iron in the discharge solids is contained in sulfate-containing compounds.
According to another embodiment of the present invention, a method for recovering silver from a mineral feed comprising silver, sulfide sulfur and iron is provided that includes the steps of pressure oxidizing the mineral feed in a reactor at a temperature of at least 190xc2x0 C. to oxidize at least 90 percent of the sulfide sulfur in the mineral feed to sulfate sulfur and to produce silver-containing discharge solids and leaching at least a portion of the discharge solids with a leach solution to dissolve at least a portion of the silver into the leach solution. According to this embodiment, the pressure oxidizing step comprises feeding an aqueous feed slurry comprising the mineral feed to the reactor, feeding a sulfate-binding material to the reactor separate from the feed slurry and withdrawing from the reactor an aqueous discharge slurry including the discharge solids.
According to another embodiment of the present invention, a method is provided for recovering silver and a non-ferrous base metal from a mineral feed comprising sulfide sulfur, iron, silver and the non-ferrous base metal, with at least a portion of the non-ferrous base metal being contained in one or more sulfide minerals. The method includes the steps of pressure oxidizing the mineral feed by feeding an aqueous feed slurry comprising the mineral feed to a reactor, feeding oxygen gas to the reactor, oxidizing at least 90 percent of the sulfide sulfur in the mineral feed to sulfate sulfur and dissolving at least 90 percent of the non-ferrous base metal from the mineral feed into aqueous liquid in the reactor. An aqueous discharge slurry comprising discharge solids and an aqueous discharge liquid is discharged from the reactor, the discharge solids including at least 90 wt. % of the silver from the mineral feed and the aqueous discharge liquid having dissolved therein at least 90 wt. % of the non-ferrous base metal from the mineral feed. After the pressure oxidizing step, discharge solids are separated from the aqueous discharge liquid and the aqueous discharge liquid is processed to remove at least a portion of the non-ferrous base metal from the aqueous discharge liquid and the discharge solids are processed to remove at least a portion of the silver from the discharge solids. Preferably, during the pressure oxidizing, the reactor is maintained at a temperature of at least 190xc2x0 C. and dissolved iron in the discharge slurry is maintained at a concentration of not greater than 1 gram of dissolved iron per liter of the discharge liquid.